The invention relates to a method for determining the absolute coordinates of an object, in which method the object is exposed to light through a projection grid, the light which is reflected by the object is registered by a sensor, and the image picked up by the sensor is evaluated. The invention also relates to a device for determining the absolute coordinates of an object, comprising projection optics for projecting a projection grid onto the object, and comprising sensor optics with a sensor for registering the light reflected by the object.
Such processes and devices with which the absolute coordinates of an object or the surface of an object can be determined are already known. Area sensors, in particular CCD sensors or CMOS sensors, are used for this purpose.
The three-dimensional geometry of the surface of an object can be determined by means of the moiré technique and by means of projected lines. Evaluation of the contour line images can be by means of a computer, for example in the so-called phase-shift method. In this arrangement, sequentially phase-shifted images, i.e. images picked up by the sensor, can be read to the computer by way of a video camera, in the so-called temporal phase-shift method. It is also possible to process the contour line images by way of Fourier evaluation. Evaluation can also be by other techniques in which only one contour line image, i.e. only one sensor image or only one video image, is necessary. Examples of this are provided in DE 39 07 430 B1 and in DE 38 43 396 B1.
However, the moiré technique only provides the relative form of the surface of the object. Generally speaking, this technique is not suitable for determining the absolute distance between the sensor, or the camera respectively, and the object from a contour line image. With the above mentioned techniques and methods it is thus not possible to determine the absolute coordinates of the surface of the object.
However, these absolute coordinates of an object are required to determine the absolute size of the object, or, in the case of stepped cross-sections, the depth of the object. Since a contour line image does not contain any information about the imaging scale, additional information beyond the contour line image is needed.
This additional information can be obtained by distance sensors or by changing the contour line distance or by moving the object or the camera. However, this involves the shifting of masses, which in turn requires a stable construction of the experimental setup and also requires a considerable amount of time. The mechanical expenditure is very substantial if some level of precision is to be achieved.
DE 40 11 406 A1 describes a method and a device for quantitative absolute measurement of the three-dimensional coordinates of a test object by means of the moiré technique. In this method, the displacement distance of a calibration body and/or of the test object is measured perpendicular to the plane of the grids (projection grid and reference grid). From EP 0 343 366 A, a further method is known in which the moiré technique is also applied.
From U.S. Pat. No. 4,802,759, a method for determining the coordinates of a point of the object is known, in which method light is projected through a projection grid and said projection grid is imaged onto the object. The pattern which results as an image of the projection grid on the object is imaged on an area-shaped, locally resolving sensor. The coordinates of the point of the object are determined by triangulation of the point, starting from the projection grid and from the sensor. However, in this method, only a single sensor image is recorded. Implementation of this method requires identification of a reference line.
A further device which uses a triangulation method is known from EP 0 181 553 A.
U.S. Pat. No. 4,564,295 discloses a method in which a grid is projected onto an object. The object is then imaged and a reference grid (moiré) is superimposed on it. For the purpose of evaluation, the reference grid is moved, or alternatively, the projection grid and the reference grid are moved synchronously, as a result of which stationary contour lines are generated on the object.
From U.S. Pat. No. 4,641,972, a method is known in which a sinusoidal grid is projected onto the object, and the object is observed at an angle. Evaluation takes place using the phase shift technique. There is no evaluation by way of triangulation.
U.S. Pat. No. 4,349,277 discloses a method in which coloured grids are projected onto the object at at least two wavelengths. Imaging is by way of a colour filter for wavelength selection onto two diode lines. Equidistant grids in various colours, which are displaced in relation to each other, are projected in parallel. Evaluation is via the ratio of intensities of the respective colours.
In several methods based on triangulation, the coded light projection is used in which the projected light of a grid strip is viewed as a plane in space. The light, which is reflected by the object and which reaches a particular sensor element, is viewed as a straight line. The object point viewed with the camera is calculated from an intersection of the straight line defined by the sensor element and the viewed light plane. Identification of the viewed light plane is achieved by way of coding using the so-called coded light projection. In this method, absolute coding of the projected plane can take place by a sequence of projected binary patterns. In addition, a phase shift can be carried out so as to enhance accuracy.
Absolute coding of the light planes can also be achieved via colour information of the projected grid. However, there is a trade-off in that this involves a dependence on the colour characteristics of the object. In a similar way, patterns are used which involve local coding, for example binary samples or colour samples. In this method, image elements which are adjacent in the sensor image are viewed, with identification of the projected light plane being achieved by way of neighbourhood observation.
U.S. Pat. No. 4,802,759 discloses a method for determining the coordinates of an object in which the object is exposed to light which is emitted through a projection grid, and with the light reflected by the object being registered by a sensor.
From U.S. Pat. No. 4,335,962, a method for determining the coordinates of an object is known in which the object is exposed to light which is emitted through a projection grid, and with the light reflected by the object being registered by a sensor. The absolute coordinates can be determined by means of a plane reference plate which is located at the margin of the imaging space.
DE 196 37 682 discloses a method for determining the spatial coordinates of objects in which three sensors can be used.
From EP 0 534 284 B1, a method for determining the absolute coordinates of an object according to the precharacterising part of claim 1 is known, in which method in a first position of the projection grid and of the sensor, a first image and evaluation are made, and in which subsequently the projection grid and/or the sensor is/are rotated by a certain angle and a second image and evaluation of the sensor are made. The absolute coordinates of the object can be determined from the evaluations.